When DMing without a battle mat I find it pain to deal with flanking. I could just ignore it but
1) More fun when players deserve tactical choices/problems.
2) I want swarms of low-level critters to be dangerous.
3) Likewise even punk 0 lvl men-at-arms should be "worth it" if you bring a posse.
When running combats without square grids / battlemats I form combatants into groups. If the players are smart they will form one group. But lacking plans and tactics they typically fight as individuals. Even with disciplined parties thieves are caught out front and fighters like to charge. Sometimes they get split across different rooms. It's abstract "these two are blocking the door", "rest are around pool". Monsters are grouped as well. "4 by 2 kobold phalanx" and "shaman with two guard dogs". With that in mind, here is a combined flanking/mob rule.
For every attacker beyond X attacking the same group they all get +1 to-hit. With X of 1, two attackers each get +1, three attackers each get +2, and so on.At most 4 same sized or 6 smaller sized attackers can beat on the same defender. 6 kobolds each with +5 to-hit should give even "Mr. plate and tower shield" something to worry over.
Where X is one of:
- count of defenders in group
- defending monster's HD, for big(er) monsters
- a swashbuckling character's lvl or perhaps base to-hit
- a number DM picks based on situation, like it's never gonna bother a Hydra